1. Field of the Invention
The present invention relates to an image forming apparatus such as printers, facsimile machines or copying machines, and particularly relates to the configuration of a support unit for supporting a carriage with respect to a guide shaft.
2. Description of the Related Art
In the background art, in order to move a carriage mounted with a recording head for use in a low-priced printer or the like forward and backward in a main scanning direction, a guide shaft having a circular shape in section is fixedly disposed in parallel with the main scanning direction, while a carriage bearing portion having a hole diameter slightly larger than the diameter of the guide shaft is formed integrally with the carriage made from synthetic resin, and the guide shaft is inserted into the bearing portion so as to support the carriage slidably thereon.
In the aforementioned configuration, however, due to the hole diameter of the carriage bearing portion slightly larger than the diameter of the guide shaft, there is a problem that looseness occurs between the hole of the carriage bearing portion and the guide shaft so that the position of the carriage bearing portion with respect to the guide shaft is not settled. Particularly, when the carriage is rotated and adjusted around the guide shaft so as to adjust the size of a gap between the recording head on the carriage and a recording medium on a platen, the gap does not change in proportion to the rotation angle of the carriage. Thus, there is a problem that the gap adjustment becomes unstable.
In order to solve the foregoing problems, JP-A-6-079944 discloses a configuration where a carriage bearing having a substantially U-lettered shape in sectional view and a plate spring to which a sliding member having a substantially triangular shape in section is attached at its front end are provided on a carriage, and two surfaces of the substantially U-lettered shape of the carriage bearing contact with the circumferential surface of a guide shaft while the guide shaft is held by the sliding member on the opposite side to the carriage bearing.
On the other hand, JP-A-7-019246 discloses a configuration where a sliding bearing including a generally planar body structure made from a thin and flat bronze sheet material is incorporated integrally on the opposite sides of a synthetic resin carriage assembly (carriage body) by insert molding. A through hole of the sliding bearing is formed into a piriform shape having two contact surfaces which are substantially flat surfaces. The contact surfaces are provided in positions of about one-thirty and ten-thirty in the upper portion of the though hole and separated from each other in the form of symmetric bows. The diameter of the lower side of the through hole is formed to be slightly larger than the diameter of a guide shaft. Thus, the two contact surfaces in the upper portion of the through hole have two linear contacts at two points on the circumferential surface of the guide shaft.